This invention relates to high intensity internal mixing machines for elastomeric materials. A batch of ingredients to be mixed is fed into a mixing chamber where two rotors rotate at predetermined speeds for predetermined mixing periods after which the mixed material is unloaded through an outlet at the bottom of the mixing chamber. With some rotor designs there has been difficulty unloading the material when the rotors are stopped in positions which tend to retain the mixed material in the mixing chamber. When the mixed material has been unloaded the unloading time has varied causing the drop door in the outlet opening hereinafter referred to as the outlet to be kept open for a period of time equal to the maximum unloading time. This has resulted in the drop door open time being increased from a normal eight to ten seconds to twenty to thirty seconds to ensure the outlet will be clear of the elastomeric material when the drop door is closed. In many cases, the mixer is unloaded in four to five seconds and therefore the lost mixing time may be as much as twenty-five seconds for each mixing cycle. Considering the high cost of these mixers, this unnecessary down time is very costly. On the other hand, when the drop door is closed with the mixed material in the outlet there is a problem with gate jams preventing the door from closing and requiring down time to clear the outlet. For these reasons, timers which have provided dwell time after opening of the outlet have not been adequate to open and close the outlet and to safely achieve the highest use of the mixers.
It has been found that a mixer construction in which sensors are built into the walls of the outlet provide signals as to the passage of the elastomeric material which can be utilized to control the closing of the outlet by movement of the drop door into the outlet opening only after the outlet is cleared. The emptying of the mixer has also been facilitated by controlling the position of at least one of the rotors when the rotation is stopped.